Missing referenced statements in GLEIF

[radix0000]

On going (nagging) issue that there are usual small (~60 missing entities - associate with one country, Lithuania, last time we checked in detail) in the GLEIF data (with some indication that there are transient spikes where it jumps to thousands - probably a separate issue where there is a mistake and which is quickly fixed). This is probably more of a to be aware off issue rather than anything else, unless we are going to more actively engage with GLEIF and try to get them to fix it.

The larger transient issues might have more impact though, and as we move to accumulating data through delta files, we need to be aware this might happen, and work out a way of managing it if it does.